Lateral epitaxial growth of two-dimensional layered semiconductor heterojunctions

Abstract

Two-dimensional layered semiconductors such as MoS₂ and WSe₂ have attracted considerable interest in recent times. Exploring the full potential of these layered materials requires precise spatial modulation of their chemical composition and electronic properties to create well-defined heterostructures. Here, we report the growth of compositionally modulated MoS₂-MoSe₂ and WS₂-WSe₂ lateral heterostructures by in situ modulation of the vapour-phase reactants during growth of these two-dimensional crystals. Raman and photoluminescence mapping studies demonstrate that the resulting heterostructure nanosheets exhibit clear structural and optical modulation. Transmission electron microscopy and elemental mapping studies reveal a single crystalline structure with opposite modulation of sulphur and selenium distributions across the heterostructure interface. Electrical transport studies demonstrate that the WSe₂-WS₂ heterojunctions form lateral p-n diodes and photodiodes, and can be used to create complementary inverters with high voltage gain. Our study is an important advance in the development of layered semiconductor heterostructures, an essential step towards achieving functional electronics and optoelectronics.

Figure 1. Schematic of lateral epitaxial growth of WS₂–WSe₂ and MoS₂–MoSe₂ heterostructures

A triangular domain of WS₂ (MoS₂) is first grown using a CVD process. The peripheral edges of the triangular domain feature unsaturated dangling bonds that function as the active growth front for the continued addition and incorporation of precursor atoms to extend the two-dimensional crystal in the lateral direction. With an in situ switch of the chemical vapour source for WSe₂ (MoSe₂) in the middle of growth, lateral heteroepitaxial growth can occur at the peripheral active growth front to form WS₂–WSe₂ (MoS₂–MoSe₂) lateral heterostructures.

Figure 2. AFM, Raman and photoluminescence characterization of WS₂–WSe₂ lateral heterostructures

a, AFM image of a triangular domain with a thickness of 1.2 nm. Inset: optical image of a triangular domain. Scale bars, 5 μm. b, Raman spectra of a heterostructure domain. The blue curve is obtained from the centre region, and shows the characteristic Raman peaks of WS₂. The green curve is obtained from the peripheral region, and shows the characteristic Raman peaks of WSe₂. c, Photoluminescence spectra of a heterostructure domain. The orange curve is obtained from the centre region and shows the characteristic photoluminescence peak of WS₂, and the red curve is obtained from the peripheral region, and shows the characteristic photoluminescence peak of WSe₂. d, Raman mapping at 419 cm⁻¹ (WS₂ A_1g signal), demonstrating that WS₂ is localized at the centre region of the triangular domain. Scale bar, 5 μm. e, Raman mapping at 256 cm⁻¹ (WSe₂ A_1g signal), demonstrating that WSe₂ is located in the peripheral region of the triangular domain. f, Composite image consisting of Raman mapping at 256 cm⁻¹ and 419 cm⁻¹, showing no apparent overlap or gap between the WS₂ and WSe₂ signals, demonstrating that the WS₂ inner triangle and WSe₂ peripheral areas are laterally connected. g,h, Photoluminescence mapping images at 665 nm and 775 nm, showing characteristic photoluminescence emission of WS₂ and WSe₂ in the centre and peripheral regions of the triangular domain, respectively. Scale bar, 5 μm. i, Composite image consisting of photoluminescence mapping at 665 nm and 775 nm, demonstrating the formation of WS₂–WSe₂ lateral heterostructures, consistent with Raman mapping studies. Scale bars in d,g apply to e,f,h,i.

Figure 3. Structural and chemical modulation in WS₂–WSe₂ lateral heterostructures

a, TEM image of a triangular domain of a WS₂–WSe₂ lateral heterostructure. Scale bar, 200 nm. b, HAADF TEM image of the heterostructure domain, showing the slightly darker inner triangle corresponding to the WS₂ region and the slightly brighter outer region corresponding to WSe₂. Scale bar, 200 nm. c, Electron diffraction pattern taken across the heterostructure interface, with each diffraction spot consisting of a pair diffraction peaks (see insets for magnified views), with indexed lattice spacings of 2.70 Å for WS₂ and 2.81 Å for WSe₂, respectively. d, High-resolution TEM image showing a highly crystalline structure with continuous lattice fringes across the WS₂–WSe₂ heterostructure interface. Scale bar, 5 nm. e–g, EDS elemental mapping images of W, S and Se atoms, showing a relatively uniform distribution of W, and clear modulation of S and Se in the triangular domain. h, Composite EDS mapping image of S and Se atoms, showing seamless lateral integration of WS₂ and WSe₂ in the heterostructure domain. Scale bars (e–h), 200 nm. i, EDS linescan profiles of S and Se distributions, showing opposite modulation across the heterostructure interface.

Figure 4. Growth and characterization of MoS₂–MoSe₂ lateral heterostructures

a, AFM and optical microscope images (inset) of a triangular MoS₂–MoSe₂ monolayer domain, with an AFM linescan indicating a step height of ∼0.8 nm, corresponding to the monolayer MoS₂–MoSe₂ domain. Scale bars, 2 μm. b, Raman spectra taken from the centre (MoS₂) and peripheral (MoSe₂) areas of a triangular heterostructure domain. c, Photoluminescence emission spectra obtained from the centre (MoS₂) and peripheral (MoSe₂) areas of a triangular heterostructure domain. d,e, Raman spectroscopy mapping images of the A_1g mode of MoS₂ at 403 cm⁻¹ (d) and A_1g mode of MoSe₂ at 235 cm⁻¹ (e). Scale bars, 2 μm. f,g, Photoluminescence mapping images of the centre MoS₂ region with emission at 680 nm (f) and the peripheral MoSe₂ region with emission at 790 nm (g). Scale bars, 2 μm. h, Low-resolution TEM image of a monolayer heterostructure. The white dotted line roughly defines the boundary between MoS₂ and MoSe₂. Scale bar, 200 nm. i,j, SAED patterns taken from the MoS₂ region (i) and the MoSe₂ region (j) of a lateral heterostructure. k, EDS linescan profiles across the MoS₂–MoSe₂ heterostructure interface, demonstrating the opposite modulations of S and Se concentration.

Figure 5. Electrical characterization and functional devices from WS₂–WSe₂ lateral heterojunctions

a, I_ds–V_ds output characteristics of a WS₂ FET at various backgate voltages (indicated in the plot) show increasing current with increasing positive gate voltage, demonstrating n-type behaviour. b, I_ds–V_ds output characteristics of a WSe₂ FET at various backgate voltages (indicated in the plot) show decreasing current with increasing positive gate voltage, demonstrating p-type behaviour. c, Gate-tunable output characteristics of a lateral WSe₂–WS₂ heterojunction p–n diode. The gate voltage varies from 80 to 20 V in steps of 10 V, as indicated. Inset: optical image of a heterojunction p–n diode device. The orange dashed line outlines the triangular heterostructure domain and the white dashed rectangle outlines the 50 nm Al₂O₃ deposited on WSe₂ to insulate the WS₂ contact electrodes. Scale bar, 2 μm. d, Experimental output (I_ds–V_ds) characteristics of the lateral WSe₂–WS₂ heterojunction p–n diode in the dark (black line) and under illumination (red line; wavelength, 514 nm; power, 30 nW). Inset: temporal photocurrent response under periodic on/off laser illumination through a mechanical chopper. e, Scanning photocurrent mapping image of the lateral WSe₂–WS₂ heterojunction p–n diode showing that the photoresponse is localized at the WSe₂–WS₂ interface and the lower-doped WS₂ region near the centre of the triangular domain. The orange dashed line outlines the triangular heterostructure domain. The yellow solid line outlines the gold electrodes. Scale bar, 2 μm. f, A CMOS inverter obtained by integrating a p-type WSe₂ and n-type WS₂ FET, showing the expected inverter function with a voltage gain as large as 24. The black curve is the output–input curve and the red curve indicates the voltage gain. Inset: Image and circuit diagram of the WSe₂–WS₂ CMOS inverter. Scale bar, 2 μm.